[ASan] Really fix zero_page_pc test for PowerPC

Summary:
As a follow-up to D6167, this patch fixes the test to account for
another difference between PowerPC and x86 systems.

On x86 if you branch to an invalid address, you get a SIGSEGV with PC
set to the invalid address (and si_addr in the siginfo struct also set
to the same address).

On PowerPC, you get a SIGSEGV with PC pointing at the branch
instruction, but si_addr set to the invalid address.

You can see this difference if you run the test case under gdb.

Reviewers: kcc, glider, samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6226